A stable Drosophila X chromosome can be destabilized by association with the unstable X chromosome (Uc) or its derivatives for only one generation. Occasionally, male flies carrying the destabilized X chromosome transmit a mutant allele not apparent in their phenotype (cryptic mutation). We are interested in learning about the origin and the underlying molecular mechanisms for transmission of cryptic mutations. In our system, two X-linked loci exhibit frequency of cryptic mutation generation and transmission. These are cut wings (ct) and forked bristles (f) loci. In a particular cross, for example, more than 2% of males carrying the destabilized X chromosomes produce at least one sperm with cryptic ct. Our experimental data are consistent with the view that a transposable element or elements are involved in transmission of the mutations. A number of separate lines of evidence indicate that the ordinary meiotic recombination process is not likely involved in generating the cryptic mutations. Suppressible insertion mutations, transposon-mediated instability, heteroduplex nature of cryptic mutant loci, and involvement of extrachromosomal replicating elements are some of the likely models we would like to test. Our experimental approach is to correlate genetic data with Southern blot transfer analysis and in situ hybridization.